In adult skeletal muscle, satellite cells reside in a niche beneath the basal lamina but outside the associated muscle fibers and are responsible for muscle growth, maintenance and repair (Bischoff, 1994; Mauro, 1961). Satellite cells are normally mitotically quiescent, but are activated (i.e. enter the cell cycle) in response to stress induced by weight bearing or by trauma such as injury (Charge and Rudnicki, 2004). The descendants of activated satellite cells, called myogenic precursor cells, undergo multiple rounds of division prior to fusion and terminal differentiation (Dhawan and Rando, 2005). Activated satellite cells also generate progeny that restore the pool of quiescent satellite cells (Collins, 2006).
The maintenance of satellite cell numbers in aged muscle after repeated cycles of degeneration and regeneration has been interpreted to support the notion that satellite cells possess an intrinsic capacity for self-renewal (Bischoff, 1994). Asymmetric distribution of Numb protein in daughters of satellite cells in cell culture has been implicated in the asymmetric generation of distinct daughter cells for self-renewal or differentiation (Conboy and Rando, 2002). Nevertheless, the precise molecular mechanisms regulating satellite cell self-renewal and differentiation remain poorly understood.
The paired-box transcription factor Pax7 plays an important role in regulating satellite cell function. Pax7 is specifically expressed in satellite cells in adult muscle and their daughter myogenic precursor cells in vivo, and primary myoblasts in vitro (Seale et al., 2000). Extensive analysis of Pax7−/− mice have confirmed the progressive ablation of the satellite cell lineage in multiple muscle groups (Kuang et al., 2006; Oustanina et al., 2004; Relaix et al., 2006; Seale et al., 2000). Small numbers of Pax7-deficient cells do survive in the satellite cell position but these cells do not express the satellite cell markers CD34 and Syn4, and arrest and die upon entering mitosis (Kuang et al., 2006; Relaix et al., 2006). Muscle in Pax7-deficient mice is reduced in size, the fibers contain approximately 50% the normal number of nuclei, and fiber diameters are significantly reduced (Kuang et al., 2006). Together, these data confirm an important role for Pax7 in regulating the productive myogenic commitment of satellite cells.
Early experiments using quail-chick chimeras suggested that satellite cells were derived from the somite (Armand et al., 1983). Recent experiments support this work and indicate that the progenitors of satellite cells originate in embryonic somites as Pax3/Pax7 expressing cells (Ben-Yair and Kalcheim, 2005; Gros et al., 2005; Kassar-Duchossoy et al., 2005; Relaix et al., 2005; Schienda et al., 2006). In addition, satellite cells may also be derived from cells associated with the embryonic vasculature including the dorsal aorta (De Angelis et al., 1999), and from other adult stem cells during regeneration (Asakura et al., 2002; LaBarge and Blau, 2002; Polesskaya et al., 2003). However, whether satellite cells are stem cells, committed progenitors or de-differentiated myoblasts (Zammit et al., 2004), remains unresolved.
Several studies have suggested that the satellite cell compartment is not a homogeneous population. Radio isotope labeling of growing rat muscle revealed that satellite cells are a mixture of 80% fast cycling cells and 20% of slow cycling “reserve cells” (Schultz, 1996). Examination of the expression of satellite cell markers CD34, M-cadherin and Myf5−nLacZ in freshly prepared myofibers has suggested that a subpopulation of satellite cells may exhibit heterogeneous expression of these markers (Beauchamp et al., 2000). However, the molecular identity of any potential subpopulations has not been defined and the prospective isolation and characterization of these cells has not been achieved.
Transplantation of the cultured primary myoblasts into regenerating muscle typically results in extensive loss of the transplanted cells, terminal differentiation of the surviving cells, and virtually no contribution to the satellite cell compartment (Beauchamp et al., 1999; El Fahime et al., 2003; Fan et al., 1996; Hodgetts et al., 2000; Qu et al., 1998; Rando and Blau, 1994). By contrast, experiments involving transplant of intact fibers carrying satellite cells (Collins et al., 2005), or prospectively isolated satellite cells (Montarras et al., 2005), has suggested that a small proportion of satellite cells has the capacity to repopulate the satellite cell compartment as well as extensively contribute to regenerating muscle. Together, these results strongly support the hypothesis that sub-laminar satellite cells in vivo are a heterogeneous population containing a minor subpopulation capable of repopulating the satellite cell niche as well as giving rise to cells committed to terminal differentiation.
There is a need in the art to identify and isolate novel stem cells. Further, there is a need in the art to employ novel stem cells in therapeutic applications. There is also a need in the art to identify nucleotide sequences and genes therefrom that are involved in growth, differentiation or both growth and differentiation of stem cells, progenitor cells, myoblasts or the like.
It is an object of the invention to overcome disadvantages of the prior art.
The above object is met by the combinations of features of the main claims, the sub-claims disclose further advantageous embodiments of the invention.